The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to purchase components for the base stations at a lower price and also wish to reduce operational expenses of the base station. Delta-sigma modulators driving a switched power amplifier have proven to meet these goals.
From A. Frappé, A. Flament, A. Kaiser, B. Stefanelli, A. Cathelin, “Design techniques for very high speed digital delta-sigma modulators aimed at all-digital RF transmitters”, IEEE 2006, p. 1113 ff a delta-sigma modulator for providing radio-frequency signals according to the WCDMA standard is known. The architecture of this known delta-sigma modulator is a third-order low pass comprising three consecutive delta-sigma modulator stages (a first delta-sigma modulator stage, a second delta-sigma modulator stage, a third delta-sigma modulator stage) and a quantizer. An output signal of the last delta-sigma modulator stage (the third delta-sigma modulator stage in this example) is fed back as a first feedback signal to a second delta-sigma modulator stage input of the second delta-sigma modulator stage and a third delta-sigma modulator stage input of the third delta-sigma modulator stage. The first feedback signal is multiplied by a first factor of 1/32 before the first feedback signal is input to the second delta-sigma modulator stage. In contrast hereto the first feedback signal is directly input to the third delta-sigma modulator stage input, which in effect is equivalent to a multiplication with a second factor having a value of one. A quantizer output signal is fed back as a second feedback signal to a first delta-sigma modulator stage input, the second delta-sigma modulator stage input, and the third delta-sigma modulator stage input. Before the second feedback signal is input to the first delta-sigma modulator stage input, the second feedback signal is multiplied by a third factor of the value ⅛. Before the second feedback signal is input to the second delta-sigma modulator stage input the second feedback signal is multiplied by a fourth factor of the value ¼. Finally, before the second feedback signal is input to the third delta-sigma modulator stage input the second feedback signal is multiplied by a fifth factor of the value ⅛.
The three delta-sigma modulator stages implement in connection with the first feedback signal and the second feedback signal a noise shaping transfer function with a notch at about 40 MHz. The chosen noise transfer function flattens the quantization noise at frequencies close to the output signal and thus extends the practically usable signal bandwidth. Two of the described third-order low pass delta-sigma modulators are combined in a digital up-converter to produce a 7,8 GS/s output signal for a digital power amplifier.
One of the issues of digital power amplifiers is that spectral distortions, measured in parameters such as ACPR, have to be within given limits to avoid interference in other channels of the same frequency band or outside the frequency band of a given communication system. As one of the possible origins of spectral distortions inter symbol interference caused by a final stage such as a digital power amplifier have been identified.